Richie told us this during his stream; right before she dies she becomes older by becoming taller, gaining breasts and bigger thighs. This was cut in the american release. Richie's 10/10 joke was, "you could say she literally blossomed".

Did you watch the Japanese version of Sonic X or the English version? In the English dub, they cut a scene where Cosmo develops breasts and hips when (pun intended) blossoming into her matured form. She also becomes taller, resembling more of the other Seedrian aliens you see in flashbacks throughout the season.